During subsea completion operations it is very difficult, and often practically impossible, to recover a casing string or pipe once it has been run into a well, owing to frictional and suction effects acting on the outer wall of the casing string or pipe. Once the casing string or pipe has landed out in the wellhead, the rig or vessel from which the operation is performed is vulnerable, because it is potentially connected mechanically to an irretrievable object. This vulnerability is increased when the weather conditions or sea state begin to deteriorate. In such circumstances the rig or vessel would need to disconnect from the wellhead; otherwise substantial damage can be sustained by the hardware and the level of danger to which the crew onboard are exposed is greatly increased. In the event of the mechanical interface between the rig or vessel and the well malfunctioning or failing to operate, a secondary system of severance or retrieval would offer significant mitigation of the dangers mentioned above.
The example embodiments provide, in a preferred form a redundant emergency release mechanism which allows the rig or vessel to disconnect with the aid of several operational features which are typically available in an offshore completion operation, preferably including rotation of the drill pipe both clockwise and anti-clockwise and the application of a pressure either by the application of bore pressure or by using a self-sealing dart deployed from the drill floor.
The state of the art is exemplified by the document U.S. Pat. No. 6,062,312-A, which discloses a running tool which has a mechanical mechanism for unlatching the running tool from a side valve tree.